The research in progress is part of a multidisciplinary project to elucidate the mechanisms of Mn(II) adsorption, oxidation, and precipitation at microbe-water interfaces and the identities (i.e., structures, compositions) and aqueous reactivities of microbially precipitated Mn(III)-and (IV)-(hydr)oxides. Microbes play key roles in the global geochemical cycling of Mn and other elements, and microbially-precipitated Mn-oxides are major sources and sinks for toxic heavy metals in the environment. Our primary objectives are to define the oxidation states, molecular structures, and compositions of microbially bound and precipitated Mn under defined experimental conditions and to characterize the effects of the species and strain of bacteria and aqueous composition on the structures, compositions, and reactivities of microbially precipitated Mn-(hydr)oxides. XAFS measurements are crucial to this project because they provide the capability to measure the oxidation states and molecular structures and compositions of Mn in aqueous species, adsorbed on bacterial surfaces, and in amorphous solids at dilute concentrations in natural samples under in-situ conditions. We have made substantial progress during the two beamtime sessions we have had to date for this proposal. Mn K-edge XANES and EXAFS measurements have been performed in-situ on samples in which microbial Mn oxidation reactions were run. Measurements were performed using an in-situ flow-through reaction cell that permits simultaneous time-resolved collection of reaction effluent. Results indicate the presence of short lived noncrystalline Mn(III) intermediate species. This conclusion contradicts previous predictions that Mn(II) oxidation proceeds via a two-electron transfer reaction directly to Mn(IV). XANES and EXAFS measurements on Mn-oxide reaction products indicates that Mn concentration and water composition control the structures of amorphous Mn(III) and (IV) precipitates that are produced by microbes.